Radio communication system for reducing interferences with respect to other communication system using close frequency band

ABSTRACT

A communication channel allocation method allocates a communication channel to communications between a mobile terminal device and a second radio base station using a second frequency band which is close to a first frequency band used by a first radio base station, by detecting a first distance between the second radio base station and the first radio base station, detecting a second distance between the second radio base station and the mobile terminal device when the first distance is less than a first threshold, and allocating a communication channel of a frequency far from the first frequency band in the second frequency band to the communications between the second radio base station and the mobile terminal device when the second distance is less than a second threshold.

This application is a con of Ser. No. 10/153,899 filed on May 24, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio communication system forproviding communication services to mobile terminal devices and acommunication channel allocation method and a communication controldevice in that system.

2. Description of the Related Art

In the simplified portable telephone known as the so called PHS(Personal Handyphone System), the mobile terminal device has a smalleroutput (10 mW) compared with the current digital cellular phone systemso that a range over which the mobile terminal device can carry outcommunications with the base station has been limited. Also, the schemeused in the existing PHS is capable of carrying out the datacommunications that are faster (32 KBPS to 64 KBPS, for example)compared with the current digital cellular phone system.

On the other hand, W-CDMA (Wideband-Code division Multiple Access) hasbeen proposed as a scheme for realizing fast data communications (64KBPS to 384 KBPS, for example).

This W-CDMA uses radio signals of 2G band (uplink: 1.92 to 1.98 GHz,downlink: 2.11 to 2.17 GHz), for example. Also, the PHS described aboveuses radio signals of 1.9 GHz band (12.89365 to 1.91945 GHz).

As such, these radio communication systems are using close frequenciesso that, in order to avoid interferences between them, the so calledguard band is provided between the frequency bands to be used by therespective radio communication systems. For example, the guard band ofabout 5 MHz is provided between the upper limit frequency of thefrequency band to be used by the PHS and the lower limit frequency ofthe frequency band to be used by the W-CDMA.

Also, the devices to be used by the respective radio communicationsystems use a filter for reducing components outside a prescribed band(channel width), so as to avoid interferences between these radiocommunication systems.

However, depending on the positional relationships among the mobileterminal devices and the base stations of both the PHS and the W-CDMA orthe like, there are cases where the components outside a prescribed bandcan be reduced sufficiently on one radio communication system but cannotbe reduced sufficiently on another radio communication system.

In particular, the signal band of the W-CDMA is about 5 MHz, forexample, so that the so called spurious components or noises aregenerated over a wide frequency region that may be extended beyond theguard band such that they can potentially cause interferences to the PHSside.

Also, the transmission power of the W-CDMA is considerably larger thanthe transmission power of the PHS so that there can also be cases wherethe so called receiver blocking occurs at the receiver of the PHS sideto lower the reception sensitivity.

In order to avoid interferences with respect to the PHS, it is possibleto consider the suppression of the generation of the spurious componentsor noises outside a prescribed band by improving the performance offilters on the W-CDMA side. However, a difference in the transmissionpower between the PHS and the W-CDMA is so large that an extremelysevere blocking characteristic (the attenuation rate in the blockingregion) would be required to the filters on the W-CDMA side if thecomponents outside a prescribed band are to be suppressed to the levelof not influencing the PHS side under any circumstances. The filter forrealizing such a characteristic would have a complicated structure usingmany elements, which would require large power consumption and size. Forthis reason, the use of such a filter is difficult especially in themobile terminal device for which demands for a smaller size and a lowerpower consumption are high.

It is also possible to consider the provision of providing a filter onthe receiver of the PHS in order to prevent mixing of noises from theW-CDMA, but for the similar reason, this provision would make the devicecomplicated and increase its cost.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide acommunication channel allocation method, a communication control device,and a radio communication system which are capable of contributing tothe reduction of interferences with respect to the other communicationsystem using radio signals of a close frequency band.

According to one aspect of the present invention there is provided acommunication channel allocation method for allocating a communicationchannel to communications between a mobile terminal device and a secondradio base station using a second frequency band which is close to afirst frequency band used by a first radio base station, thecommunication channel allocation method comprising the steps of:detecting a first distance between the second radio base station and thefirst radio base station; detecting a second distance between the secondradio base station and the mobile terminal device when the firstdistance is less than a first threshold; and allocating a communicationchannel of a frequency far from the first frequency band in the secondfrequency band to the communications between the second radio basestation and the mobile terminal device when the second distance is lessthan a second threshold.

According to another aspect of the present invention there is provided acommunication channel allocation method for allocating a communicationchannel to communications between a mobile terminal device and a secondradio base station using a second frequency band which is close to afirst frequency band used by a first radio base station, thecommunication channel allocation method comprising the steps of:detecting a distance between the first radio base station and the mobileterminal device; and allocating a communication channel of a frequencyfar from the first frequency band in the second frequency band to thecommunications between the second radio base station and the mobileterminal device when the distance is less than a prescribed threshold.

According to another aspect of the present invention there is provided acommunication control device for controlling a communication channel tobe used for communications between a mobile terminal device and a secondradio base station using a second frequency band which is close to afirst frequency band used by a first radio base station, thecommunication control device comprising: a first distance detection unitconfigured to detect a first distance between the second radio basestation and the first radio base station; a second distance detectionunit configured to detect a second distance between the second radiobase station and the mobile terminal device when the first distance isless than a first threshold; and a channel allocation unit configured toallocate a communication channel of a frequency far from the firstfrequency band in the second frequency band to the communicationsbetween the second radio base station and the mobile terminal devicewhen the second distance is less than a second threshold.

According to another aspect of the present invention there is provided acommunication control device for controlling a communication channel tobe used for communications between a mobile terminal device and a secondradio base station using a second frequency band which is close to afirst frequency band used by a first radio base station, thecommunication control device comprising: a distance detection unitconfigured to detect a distance between the first radio base station andthe mobile terminal device; and a channel allocation unit configured toallocate a communication channel of a frequency far from the firstfrequency band in the second frequency band to the communicationsbetween the second radio base station and the mobile terminal devicewhen the distance is less than a prescribed threshold.

According to another aspect of the present invention there is provided acommunication channel allocation method for allocating a communicationchannel to communications between one mobile terminal device and asecond radio base station using a second frequency band which is closeto a first frequency band used by a first radio base station, thecommunication channel allocation method comprising the steps of:controlling a received power of signals from the one mobile terminaldevice at the second radio base station to be constant; obtainingallocation rates for channels to be used for communications between thesecond radio base station and the one mobile terminal device accordingto a total number of mobile terminal devices that are carrying outcommunications with the second radio base station and a total receivedpower of signals other than signals from the one mobile terminal device;obtaining a channel switching distance to be a criterion for switchingcommunication channels, according to the allocation rates; detecting adistance between the second radio base station and the one mobileterminal device; and allocating a communication channel of a frequencyfar from the first frequency band in the second frequency band to thecommunications between the second radio base station and the mobileterminal device when the distance is greater than the channel switchingdistance.

According to another aspect of the present invention there is provided acommunication control device for controlling a communication channel tobe used for communications between one mobile terminal device and asecond radio base station using a second frequency band which is closeto a first frequency band used by a first radio base station, thecommunication control device comprising: a received power control unitconfigured to control a received power of signals from the one mobileterminal device at the second radio base station to be constant; anallocation rate calculation unit configured to obtain allocation ratesfor channels to be used for communications between the second radio basestation and the one mobile terminal device according to a total numberof mobile terminal devices that are carrying out communications with thesecond radio base station and a total received power of signals otherthan signals from the one mobile terminal device; a distance calculationunit configured to obtain a channel switching distance to be a criterionfor switching communication channels, according to the allocation rates;a distance detection unit configured to detect a distance between thesecond radio base station and the one mobile terminal device; and achannel allocation unit configured to allocate a communication channelof a frequency far from the first frequency band in the second frequencyband to the communications between the second radio base station and themobile terminal device when the distance is greater than the channelswitching distance.

Other features and advantages of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a radio communicationsystem according to the first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a radio controldevice in the radio communication system of FIG. 1.

FIG. 3 is a diagram showing exemplary communication channels to be usedin the radio communication system of FIG. 1.

FIG. 4 is a flow chart for one exemplary communication channelallocation processing in the radio communication system of FIG. 1.

FIG. 5 is a flow chart for another exemplary communication channelallocation processing in the radio communication system of FIG. 1.

FIG. 6 is a diagram showing a configuration of a radio communicationsystem according to the second embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of a radio controldevice in the radio communication system of FIG. 6.

FIG. 8 is a diagram showing exemplary communication channels to be usedin the radio communication system of FIG. 6.

FIG. 9 is a diagram showing a channel switching distance to be used inthe radio communication system of FIG. 6.

FIG. 10 is a flow chart for an exemplary communication channelallocation processing in the radio communication system of FIG. 6.

FIG. 11 is a flow chart for an exemplary communication channel switchingprocessing in the radio communication system of FIG. 6.

FIG. 12 is a diagram showing one exemplary configuration of the radiocommunication system according to the first embodiment of the presentinvention.

FIG. 13 is a diagram showing another exemplary configuration of theradio communication system according to the first embodiment of thepresent invention.

FIG. 14 is a diagram showing another exemplary configuration of theradio communication system according to the first embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 to FIG. 5, one embodiment of a radiocommunication system according to the present invention will bedescribed in detail.

The present invention is applicable to the communication channelallocation, the communication control, etc., in a communication systemoperated under an environment in which another communication systemusing the close frequencies is existing, for example.

As shown in FIG. 1, for example, this radio communication system 20provides communication services by using radio signals in a frequencyband close to that of a radio communication system 10 having a pluralityof base stations 11 ₁ to 11 _(n) for providing the communicationservices, for example, and a mobile terminal device 12 for utilizing thecommunication services provided by the base stations 11 ₁ to 11 _(n).

The radio communication system 10 is the PHS (Personal HandyphoneSystem), for example, which uses radio signals of 1.9 GHz band (1.89365to 1.91945 GHz), for example, for the communications between the basestations 11 ₁ to 11 _(n) and the mobile terminal device 12. Also, thisradio communication system 10 uses the TDMA (Time Division MultipleAccess) scheme in order to carry out communications between one basestation and a plurality of mobile terminal devices. For channels forcarrying out communications by such a TDMA scheme, a frequency band of300 KHz is used per one channel, for example.

For the base stations 11 ₁ to 11 _(n), respective areas (cells) 13 ₁ to13 _(n) are allocated. Also, each one of the base stations 11 ₁ to 11_(n) is connected to a radio communication control unit 14 through acommunication line.

Each one of the base stations 11 ₁ to 11 _(n) provides a connectionservice for the Internet, a wired communication network, another radiocommunication network, etc., for example, with respect to the mobileterminal device 12 within a corresponding one of the cells 13 ₁ to 13_(n), through the radio communication control unit 14.

Also, the radio communication system 20 has a plurality of base stations21 ₁ to 21 _(m) for providing the communication services, a mobileterminal device 22 for utilizing the communication services provided bythe base stations 21 ₁ to 21 _(m), and a radio communication controlunit 24 for carrying out a control of radio communications between thebase stations 21 ₁ to 21 _(m) and the mobile terminal device 22.

The radio communication system 20 is a portable telephone system of theW-CDMA (Wideband-Code Division Multiple Access) scheme, for example,which uses radio signals of 2 GHz band (uplink: 1.92 to 1.98 GHz,downlink: 2.11 to 2.17 GHz), for example, for the communications betweenthe base stations 21 ₁ to 21 _(m) and the mobile terminal device 22.Also, this radio communication system 20 uses the CDMA (Code DivisionMultiple Access) scheme in order to carry out communications using aplurality of channels, for which a frequency band of 5 MHz is used perone channel, for example. A plurality of such frequency bands areprovided for each service provider of the radio communication system,for example.

Note that the transmission output of the mobile terminal device 12 ofthe radio communication system 10 is about 10 mW, for example, which isextremely small compared with the transmission output of the mobileterminal device 22 of the radio communication system 20.

For the base stations 21 ₁ to 21 _(m), respective areas (cells) 23 ₁ to23 _(m) are allocated. Also, each one of the base stations 21 ₁ to 21_(m) is connected to the radio communication control unit 24 through awired or wireless communication line.

Each one of the base stations 21 ₁ to 21 _(m) provides a connectionservice for the Internet, a wired communication network, another radiocommunication network, etc., for example, with respect to the mobileterminal device 22 within a corresponding one of the cells 23 ₁ to 23_(m), through the radio communication control unit 24.

At least a part of the cells 13 ₁ to 13 _(n) corresponding to the basestations 11 ₁ to 11 _(n) described above is overlapping with a cell 23 ₁corresponding to the base station 21 ₁.

Also, in FIG. 1, one mobile terminal device 12 and one mobile terminaldevice 22 are shown for the sake of simplicity, but the number of themobile terminal devices 12 and 22 are not limited and it is possible touse a plurality of mobile terminal devices 12 and/or a plurality ofmobile terminal devices 22.

The radio communication control unit 24 has a configuration shown inFIG. 2, for example, which has a memory 25 for storing information(location information) indicating a location of the mobile terminaldevice 22 of each user and information (base station data) such aslocations of the base stations 11 ₁ to 11 _(n) of the radiocommunication system 10, a base station control unit 26 for controllingoperations of the base stations 21 ₁ to 21 _(m), an exchange unit 27 forcontrolling communications between the base stations 21 ₁ to 21 _(m) anda network 30 or the like, a radio channel control unit 28 forcontrolling (uplink and downlink) communication channels to be used forcommunications between the base stations 21 ₁ to 21 _(m) and the mobileterminal device 22, and a call processing control unit 29 for carryingout a control of a call termination for the mobile terminal device 22 ora call originating from the mobile terminal device 22.

The memory 25 stores a terminal location table 25 a for indicatinglocations of the mobile terminal devices 22 that are using the radiocommunication system 20, and a base station information table 25 b forindicating information such as locations and communication channels inuse of the base stations 11 ₁ to 11 _(n) of the radio communicationsystem 10. Note that the memory 25 also stores a table for indicatinglocations of the base stations 21 ₁ to 21 _(m) and correspondences tothe base stations 11 ₁ to 11 _(n) that have the cells 13 ₁ to 13 _(n)overlapping with the cells 23 ₁ to 23 _(m), for each one of the basestations 21 ₁ to 21 _(m).

The location information of each mobile terminal device 22 stored in theterminal location table 25 a is information indicating a location ofthat mobile terminal device 22 that is supplied from that mobileterminal device 22, for example. The mobile terminal device 22 obtainsits own location according to strengths of radio signals from aplurality of base stations 21 ₁ to 21 _(m) and locations of these basestations 21 ₁ to 21 _(m), and supplies it to the radio communicationcontrol unit 24 along with an identification information (user ID)assigned to that mobile terminal device 22. The radio communicationcontrol unit 24 stores the supplied identification information andlocation information into the terminal location table 25 a through thebase station control unit 26 and the radio channel control unit 28.

Alternatively, it is also possible to provide a location detection unitsuch as the so called GPS (Global Positioning System) to the mobileterminal device 22 and supplies the location of the mobile terminaldevice 22 detected by this location detection unit to the radiocommunication control unit 24 similarly as described above. Theinformation indicating the location from the mobile terminal device 22is supplied to the radio communication control unit 24 at a prescribedtime interval, for example, such that the location of the mobileterminal device 22 stored in the terminal location table 25 a isregularly updated accordingly.

The radio channel control unit 28 is connected with the radiocommunication control unit 14 of the radio communication system 10through a network 40, for example. Also, the this radio channel controlunit 28 is connected with a receiver 50 for receiving radio signals fromthe base stations 11 ₁ to 11 _(n) that constitute the radiocommunication system 10. This receiver 50 detects a cell ID, a location,etc., of each one of the base stations 11 ₁ to 11 _(n) according to theradio signals received from each one of the base stations 11 ₁ to 11_(n), and supplies them to the radio channel control unit 28. The radiochannel control unit 28 stores the supplied cell ID, location, etc.,into the base station information table 25 b described above as the basestation data.

Note that, in order to detect the locations of the base stations 11 ₁ to11 _(n), the receiver 50 may detect only the cell IDs of the basestations 11 ₁ to 11 _(n) and the radio channel control unit 28 mayacquire the information indicating the locations and the like of thebase stations 11 ₁ to 11 _(n) corresponding to these cell IDs from theradio communication control unit 14 or the like via the network 40. Itis also possible for the radio channel control unit 28 to simply acquirethe information indicating the locations and the like of the basestations 11 ₁ to 11 _(n) from the radio communication control unit 14instead.

Now, as shown in FIG. 3, for example, the frequency band to be used forthe downlink (transmission from the base station to the mobile terminaldevice) in the radio communication system 20 is separated from thefrequency band to be used by the PHS but the lower limit frequency ofthe frequency band to be used for the uplink (transmission from themobile terminal device to the base station) is close to the upper limitfrequency of the frequency band to be used by the PHS. Note that FIG. 3compares the transmission powers of the mobile terminal device 12 andthe mobile terminal device 22, which are different from the signalstrengths from the mobile terminal devices 12 and 22 at the basestations 11 ₁ to 11 _(n).

As shown in FIG. 3, the radio communication system 10 is made to becapable of appropriately selecting one of communication channels in thefrequency band of the 1.9 GHz band described above, for example, andusing it for communications between the base stations 11 ₁ to 11 _(n)and the mobile terminal device 12. Also, the radio communication system20 is made to be capable of appropriately selecting a communicationchannel among k channels (chA1, chA2, chA3, . . . , chAk-1, chAk) witheach having 5 MHz bandwidth, and using it for communications between thebase stations 21 ₁ to 21 _(m) and the mobile terminal device 22.

Also, as shown in FIG. 3, a frequency band (guard band) that is not tobe used by either radio communication system is provided between thefrequency band to be used by the radio communication system 10 and thefrequency band to be used by the radio communication system 20. Thisguard band has a bandwidth of 5 MHz, for example.

Also, as described above, the transmission output of the mobile terminaldevice 12 of the radio communication system 10 is extremely smallcompared with the transmission output of the mobile terminal device 22of the radio communication system 20, so that there are cases where thelarge noises are caused to the radio communication system 10 side evenwhen the components outside the prescribed frequency band are attenuatedto a sufficient level at the radio communication system 20 side.

Also, because the transmission power of the mobile terminal device 22 isextremely larger than the transmission power of the mobile terminaldevice 12, there are cases where the so called receiver blocking occursat the receiver of the base stations 11 ₁ to 11 _(n) to lower thereference sensitivity, depending on conditions.

For this reason, this radio communication system 20 is made to controlthe communication channel to be used for the uplink from the mobileterminal device 22 to the base station 21 in order to reduce theinfluence that can be given to the radio communication system 10 side.

The strength (power) of the components outside the frequency band fromthe mobile terminal device 22 that are observed as noises at the basestations 11 ₁ to 11 _(n) will vary according to a distance between themobile terminal device 22 and each of the base stations 11 ₁ to 11 _(n)and a difference in frequency between the communication channel used bythe mobile terminal device 22 and the communication channel used by eachof the base stations 11 ₁ to 11 _(n).

More specifically, when the distance between the mobile terminal device22 and each of the base stations 11 ₁ to 11 _(n) becomes large, thestrength of the components outside the frequency band from the mobileterminal device 22 is lowered due to the spatial propagation loss. Also,when the difference in frequency between the communication channel usedby the mobile terminal device 22 and the communication channel used byeach of the base stations 11 ₁ to 11 _(n) becomes large, the strength ofthe components outside the frequency band that are observed as noises atthe base stations 11 ₁ to 11 _(n) is lowered. This is due to the factthat the strength of the spurious components such as higher harmoniccomponents, the intermodulation components, etc., that are caused by thenon-linearity of the amplifier of the radio communication device or thelike becomes lower as they are more separated from the carrierfrequency.

For this reason, the radio channel control unit 28 is made to be capableof obtaining a distance between the mobile terminal device 22 and eachof the base stations 11 ₁ to 11 _(n) and a difference in frequencybetween the communication channel used by each of the base stations 11 ₁to 11 _(n) and the communication channel used for the uplink from themobile terminal device 22, according to the terminal location table 25 aand the base station information table 25 b stored in the memory 25described above and the frequency of the communication channel for theuplink from the mobile terminal device 22 that is managed separately. Inaddition, the radio channel control unit 28 judges whether the detecteddistance and frequency difference are at such a level to causeinterferences or not, and executes the processing for allocating thecommunication channel to be used for the uplink from the mobile terminaldevice 22 according to this judgement result.

Now, when the base stations 11 ₁ to 11 _(n) are located within aprescribed distance from the base station 21 _(x), there can be caseswhere it suffices to judge whether interferences are caused or not byusing distances to the mobile terminal devices 22 that are obtained byapproximation for assuming that the base stations 11 ₁ to 11 _(n) arelocated at the location of that base station 21 _(x).

Also, as for the difference in frequency between the communicationchannel used by each of the base stations 11 ₁ to 11 _(n) and thecommunication channel used for the uplink from the mobile terminaldevice 22, because the channel width of the communication channel of theradio communication system 20 is extremely larger than the channel widthof the communication channel of the radio communication system 10, therecan be cases where it suffices to judge whether interferences are causedor not by accounting only the communication channel used for the uplinkfrom the mobile terminal device 22, without accounting for thecommunication channel used by each of the base stations 11 ₁ to 11 _(n).

By simplifying parameters for judging whether interferences are causedor not in this way, it is possible to reduce the load of thecommunication channel allocation processing at the radio communicationcontrol unit 24.

In this communication channel allocation processing, as shown in FIG. 4,for example, when a call from the mobile terminal device 22 with respectto the base station 21 ₁ occurs at the step S1, the radio channelcontrol unit 28 checks whether any of the base stations 11 ₁ to 11 _(n)of the radio communication system 10 exists within the cell 23 ₁ of thatbase station 21 ₁ or not, by referring to the base station informationtable 25 b in the memory 25 (step S2). When none of the base stations 11₁ to 11 _(n) exists within that cell 23 ₁, the processing proceeds tothe step S3, where an arbitrary communication channel among the abovedescribed channels (chA1, chA2, . . . ) is allocated as a frequency ofthe uplink from the mobile terminal device 22, and the communicationchannel allocation processing is finished.

On the other hand, when any of the base stations 11 ₁ to 11 _(n) of theradio communication system 10 exists within the cell 23 ₁ of that basestation 21 ₁, the processing proceeds to the step S4, where whether theinformation indicating locations of the corresponding base stations 11 ₁to 11 _(n) is recorded in the base station information table 25 b or notis checked. When the information indicating locations of these basestations 11 ₁ to 11 _(n) is recorded in the base station informationtable 25 b, the processing proceeds to the step S7.

When the information indicating locations of the corresponding basestations 11 ₁ to 11 _(n) is not recorded in the base station informationtable 25 b, the radio channel control unit 28 acquires the informationindicating locations of the corresponding base stations 11 ₁ to 11 _(n)from the radio communication control unit 14 via the network 40, forexample (step S5) and stores it into the base station information table25 b (step S6), and the processing proceeds to the step S7.

Note that, instead of acquiring the information indicating locations ofthe corresponding base stations 11 ₁ to 11 _(n) from the radiocommunication control unit 14 via the network 40 at the step S5, thelocations of the base stations 11 ₁ to 11 _(n) may be acquired accordingto the cell IDs or the like in the signals received from the basestations 11 ₁ to 11 _(n) by the receiver 50, for example. Also, theprocessing of these steps S4 to S6 is executed for each one of the basestations 11 ₁ to 11 _(n) within the cell 23 ₁. In this way, a state inwhich the information indicating locations of all of the base stations11 ₁ to 11 _(n) within the cell 23 ₁ is stored in the base stationinformation table 25 b is realized before the step S7 is executed.

At the step S7, the radio channel control unit 28 obtains a distance(D11) between the base station 21 ₁ and each one of the base stations 11₁ to 11 _(n) within the cell 23 ₁ from the locations of the basestations 11 ₁ to 11 _(n) in the base station information table 25 b andthe separately stored information indicating the location of the basestation 21 ₁. Next, at the step S8, the radio channel control unit 28compares the distance (D11) between the base station 21 ₁ and each oneof the base stations 11 ₁ to 11 _(n) within the cell 23 ₁ with aprescribed distance (dis). When all of the distances (D11) between thebase station 21 ₁ and the base stations 11 ₁ to 11 _(n) within the cell23 ₁ are greater than or equal to the prescribed distance (dis), theprocessing proceeds to the step S3, where an arbitrary communicationchannel is allocated to the uplink from the mobile terminal device 22,and the communication channel allocation processing is finished.

On the other hand, when the distance (d11) between the base station 21 ₁and any one of the base stations 11 ₁ to 11 _(n) within the cell 23 ₁ isless than the prescribed distance (dis), the processing proceeds to thestep S9, where the radio channel control unit 28 checks whether thelocation information of the mobile terminal device 22 is stored in theterminal location table 25 a or not. When the location information ofthe mobile terminal device 22 is stored in the terminal location table25 a, the processing proceeds to the step S12, whereas otherwise theprocessing proceeds to the step S10.

In the latter case, the radio channel control unit 28 acquires thelocation of the mobile terminal device 22 (step S10) and stores it intothe terminal location table 25 a (step S11), and the processing proceedsto the step S12.

At the step S12, the radio channel control unit 28 calculates a distance(D22) between the base station 21 ₁ and the mobile terminal device 22.

After that, the radio channel control unit 28 selects a communicationchannel to be allocated to the uplink from the mobile terminal device 22at the step S13 and subsequent steps.

Here, it is assumed that 1≦s≦l≦k and d22min≦d22max, where s and l arearbitrary integers, which are numbers to be set up in order to make thechannel set up easier by dividing the channels that can be used by theradio communication system 20 into three regions. Also, d22max is adistance between the base station 21 ₁ and the mobile terminal device 22under the condition of not causing interferences to those base stations11 ₁ to 11 _(n) for which the distance (D11) from the base station 21 ₁is less than the prescribed distance (dis) even when the communicationchannel closest to the frequency band to be used by the radiocommunication system 10 (chA1 in FIG. 3 described above) is used as thecommunication channel for the uplink from the mobile terminal device 22,and this d22max is obtained by experiments, for example.

First, at the step S13, the radio channel control unit 28 judges whetherthe distance (D22) between the base station 21 ₁ and the mobile terminaldevice 22 obtained at the step S12 is greater than a prescribedthreshold (d22max) or not.

When the distance (D22) between the base station 21 ₁ and the mobileterminal device 22 is greater than the prescribed threshold (d22max), itis possible to consider that those base stations 11 ₁ to 11 _(n) forwhich the distance (D11) from the base station 21 ₁ is less than theprescribed distance (dis) will not be receiving interferences by thecommunication channel for the uplink from the mobile terminal device 22.For this reason, the radio channel control unit 28 allocates availablechannel among the communication channels (chA1 to chAs) close to thefrequency band to be used by the radio communication system 10 as thecommunication channel for the uplink from the mobile terminal device 22through the base station control unit 26 at the step S14, and thecommunication channel allocation processing is finished. The allocatedcommunication channel is notified to the base station control unit 26and the transmission for the uplink from the mobile terminal device 22is started.

On the other hand, when the distance (D22) between the base station 21 ₁and the mobile terminal device 22 is less than or equal to theprescribed threshold (d22max), the radio channel control unit 28 judgeswhether the distance (D22) between the base station 21 ₁ and the mobileterminal device 22 obtained at the step S12 is less than or equal to aprescribed threshold (d22min) or not.

When the distance (D22) between the base station 21 ₁ and the mobileterminal device 22 is less than or equal to the prescribed threshold(d22min), if the communication channel close to the frequency band to beused by the radio communication system 10 is allocated as thecommunication channel for the uplink from the mobile terminal device 22,it is possible to consider that those base stations 11 ₁ to 11 _(n) forwhich the distance (D11) from the base station 21 ₁ is less than theprescribed distance (dis) will be receiving interferences. For thisreason, the radio channel control unit 28 allocates available channelamong the communication channels (chAl to chAk) far from the frequencyband to be used by the radio communication system 10 as thecommunication channel for the uplink from the mobile terminal device 22through the base station control unit 26 at the step S16, and thecommunication channel allocation processing is finished. The allocatedcommunication channel is notified to the base station control unit 26and the transmission for the uplink from the mobile terminal device 22is started.

On the other hand, when both of the conditions of the step S13 and thestep S15 described above are not satisfied, i.e., when the distance(D22) between the base station 21 ₁ and the mobile terminal device 22 isgreater than d22min and less than or equal to d22max, the processingproceeds to the step S17 where the radio channel control unit 28allocates available channel among the communication channels (chAs+1 tochAl−1) other than those that can be allocated by the step S14 and thestep S16 described above as the communication channel for the uplinkfrom the mobile terminal device 22 through the base station control unit26, and the communication channel allocation processing is finished. Theallocated communication channel is notified to the base station controlunit 26 and the transmission for the uplink from the mobile terminaldevice 22 is started.

Note that when the above described d22max and d22min are equal, theabove described s and l are set to be s=l so as to divide thecommunication channels that can be used for the uplink from the mobileterminal device 22 into two. In this case, either one of the conditionsof the step S13 and the step S15 described above is always satisfied, sothat the processing of the step S17 will not be executed.

Also, the communication channel allocation processing from the step S1to the step S17 of FIG. 4 described above will be executed for eachmobile terminal device 22 separately. In this way, the appropriatecommunication channels for the uplink can be set up even when aplurality of mobile terminal devices 22 exist.

As described above, in this radio communication system 20, when the basestations 11 ₁ to 11 _(n) exist within a prescribed distance (D11) fromthe base station 21 ₁, if the distance between the base station 21 ₁ andthe mobile terminal device 22 is less than or equal to a prescribeddistance (d22min), the communication channel far from the frequency bandthat can be used by the radio communication system 10 is allocated asthe communication channel for the uplink from the mobile terminal device22.

Consequently, in this radio communication system 20, the reduction ofthe interferences caused to the base stations 11 ₁ to 11 _(n) of theradio communication system 10 can be realized easily.

Also, in this radio communication system 20, there is no need torequired an unnecessarily severe characteristic to the filter forsuppressing the components outside the prescribed frequency band, sothat there is no need to make the configuration of the mobile terminaldevice 22 or the base stations 11 ₁ to 11 _(n) unnecessarilycomplicated. For this reason, it is possible to contribute to thereduction of the size and the power consumption of these devices. It isalso possible to prevent an unnecessary increase of the cost.

Note that the above description is directed to the processing forallocating the communication channel to be used for the uplink from onemobile terminal device 22 to one of the base stations 21 ₁ to 21 _(m),but there can also be cases for transmitting the uplinks from one mobileterminal device 22 to a plurality of the base stations 21 ₁ to 21 _(m).In such a case, the communication channel allocation processing from thestep S1 to the step S17 of FIG. 4 described above will be executed foreach uplink.

Note also that the above description is directed to the case ofcontrolling the allocation of the communication channel to be used forthe uplink from the mobile terminal device 22 according to the distancebetween the base station 21 ₁ and the mobile terminal device 22 when thebase stations 11 ₁ to 11 _(n) exist within a prescribed distance (dis)from the base station 21 ₁. By processing according to the distancebetween the base station 21 ₁ and the mobile terminal device 22 in thisway, the processing load becomes smaller compared with the case ofobtaining the distance between each one of the base stations 11 ₁ to 11_(n) and the mobile terminal device 22.

However, from a viewpoint of improving the spatial utilizationefficiency of radio signals, there can be cases where it is preferableto obtain the distance between each one of the base stations 11 ₁ to 11_(n) and the mobile terminal device 22 and control the allocation of thecommunication channel to be used for the uplink from the mobile terminaldevice 22 according to these distances.

In this communication channel allocation processing, as shown in FIG. 5,for example, when a call from the mobile terminal device 22 with respectto the base station 21 ₁ occurs at the step S21, the radio channelcontrol unit 28 checks whether any of the base stations 11 ₁ to 11 _(n)of the radio communication system 10 exists within the cell 23 ₁ of thatbase station 21 ₁ or not, by referring to the base station informationtable 25 b in the memory 25 (step S22). When none of the base stations11 ₁ to 11 _(n) exists within that cell 23 ₁, the processing proceeds tothe step S23, where an arbitrary communication channel among the abovedescribed channels (chA1, chA2, . . . ) is allocated as a frequency ofthe uplink from the mobile terminal device 22, and the communicationchannel allocation processing is finished.

On the other hand, when any of the base stations 11 ₁ to 11 _(n) of theradio communication system 10 exists within the cell 23 ₁ of that basestation 21 ₁, the processing proceeds to the step S24, where whether theinformation indicating locations of the corresponding base stations 11 ₁to 11 _(n) is recorded in the base station information table 25 b or notis checked. When the information indicating locations of these basestations 11 ₁ to 11 _(n) is recorded in the base station informationtable 25 b, the processing proceeds to the step S27.

When the information indicating locations of the corresponding basestations 11 ₁ to 11 _(n) is not recorded in the base station informationtable 25 b, the radio channel control unit 28 acquires the informationindicating locations of the corresponding base stations 11 ₁ to 11 _(n)from the radio communication control unit 14 via the network 40, forexample (step S25), stores it into the base station information table 25b (step S26), and the processing proceeds to the step S27.

Note that, instead of acquiring the information indicating locations ofthe corresponding base stations 11 ₁ to 11 _(n) from the radiocommunication control unit 14 via the network 40 at the step S25, thelocations of the base stations 11 ₁ to 11 _(n) may be acquired accordingto the cell IDs or the like in the signals received from the basestations 11 ₁ to 11 _(n) by the receiver 50, for example. Also, theprocessing of these steps S24 to S26 is executed for each one of thebase stations 11 ₁ to 11 _(n) within the cell 23 ₁. In this way, a statein which the information indicating locations of all of the basestations 11 ₁ to 11 _(n) within the cell 23 ₁ is stored in the basestation information table 25 b is realized before the step S27 isexecuted, similarly as in the communication channel allocationprocessing shown in FIG. 4 described above.

At the step S27, the radio channel control unit 28 checks whether thelocation information of the mobile terminal device 22 is stored in theterminal location table 25 a or not. When the location information ofthe mobile terminal device 22 is stored in the terminal location table25 a, the processing proceeds to the step S30.

When the location information of the mobile terminal device 22 is notstored in the terminal location table 25 a, the radio channel controlunit 28 requests the location information of the mobile terminal device22 (step S28) and stores the location information supplied in responseto this into the terminal location table 25 a (step S29), and theprocessing proceeds to the step S30. The request for the locationinformation of the mobile terminal device 22 may be made directly to themobile terminal device 22 through the base station control unit 26, forexample, or may be made to a management server device or the like formanaging the location of each mobile terminal device 22 through thenetwork 30, for example.

When the locations of the base stations 11 ₁ to 11 _(n) and the mobileterminal device 22, the radio channel control unit 28 obtains a distance(D33) between each one of the base stations 11 ₁ to 11 _(n) and themobile terminal device 22 at the step S30.

After the distance (D33) between each one of the base stations 11 ₁ to11 _(n) and the mobile terminal device 22 is obtained in this way, theradio channel control unit 28 selects a communication channel to beallocated to the uplink from the mobile terminal device 22 at the stepS31 and subsequent steps.

Here, it is assumed that 1≦u≦v≦k and d33min≦d33max, where u and v arearbitrary integers, which are numbers to be set up in order to make thechannel set up easier by dividing the channels that can be used by theradio communication system 20 into three regions. Also, d33max is adistance between each one of the base stations 11 ₁ to 11 _(n) and themobile terminal device 22 under the condition of not causinginterferences to the base stations 11 ₁ to 11 _(n) even when thecommunication channel closest to the frequency band to be used by theradio communication system 10 (chA1 in FIG. 3 described above) is usedas the communication channel for the uplink from the mobile terminaldevice 22, and this d33max is obtained by experiments, for example.

First, at the step S31, the radio channel control unit 28 judges whetherall of the distances (D33) between the base stations 11 ₁ to 11 _(n) andthe mobile terminal device 22 obtained at the step S30 are greater thana prescribed threshold (d33max) or not.

When all of the distances (D33) between the base stations 11 ₁ to 11_(n) and the mobile terminal device 22 are greater than the prescribedthreshold (d33max), it is possible to consider that all the basestations 11 ₁ to 11 _(n) within the cell 23 ₁ of the base station 21 ₁will not be receiving interferences by the communication channel for theuplink from the mobile terminal device 22. For this reason, the radiochannel control unit 28 allocates available channel among thecommunication channels (chA1 to chAu) close to the frequency band to beused by the radio communication system 10 as the communication channelfor the uplink from the mobile terminal device 22 at the step S32, andthe communication channel allocation processing is finished. Theallocated communication channel is notified to the base station controlunit 26 and the transmission for the uplink from the mobile terminaldevice 22 is started.

On the other hand, when any of the distances (D33) between the basestations 11 ₁ to 11 _(n) and the mobile terminal device 22 is less thanor equal to the prescribed threshold (d33max), the radio channel controlunit 28 Judges whether any of the distances (D33) between the basestations 11 ₁ to 11 _(n) and the mobile terminal device 22 obtained atthe step S30 is less than or equal to a prescribed threshold (d33min) ornot.

When any of the distances (D33) between the base stations 11 ₁ to 11_(n) and the mobile terminal device 22 is less than or equal to theprescribed threshold (d33min), if the communication channel close to thefrequency band to be used by the radio communication system 10 isallocated as the communication channel for the uplink from the mobileterminal device 22, it is possible to consider that this one of the basestations 11 ₁ to 11 _(n) for which the distance (D33) from the mobileterminal device 22 is less than the prescribed distance (d33min) will bereceiving interferences by the communication channel from the mobileterminal device. For this reason, the radio channel control unit 28allocates available channel among the communication channels (chAv tochAk) far from the frequency band to be used by the radio communicationsystem 10 as the communication channel for the uplink from the mobileterminal device 22 at the step S34, and the communication channelallocation processing is finished. The allocated communication channelis notified to the base station control unit 26 and the transmission forthe uplink from the mobile terminal device 22 is started.

On the other hand, when both of the conditions of the step S31 and thestep S33 described above are not satisfied, i.e., when all the distances(D33) between the base stations 11 ₁ to 11 _(n) and the mobile terminaldevice 22 are greater than d33min and any of the distances (D33) betweenthe base stations 11 ₁ to 11 _(n) and the mobile terminal device 22 isless than or equal to d33max, the processing proceeds to the step S35where the radio channel control unit 28 allocates available channelamong the communication channels (chAu+1 to chAv−1) other than thosethat can be allocated by the step S32 and the step S34 described aboveas the communication channel for the uplink from the mobile terminaldevice 22, and the communication channel allocation processing isfinished. The allocated communication channel is notified to the basestation control unit 26 and the transmission for the uplink from themobile terminal device 22 is started.

Also, the communication channel allocation processing from the step S21to the step S35 of FIG. 5 described above will be executed for eachmobile terminal device 22 separately. In this way, the appropriatecommunication channels for the uplink can be set up even when aplurality of mobile terminal devices 22 exist.

As described above, in this communication channel allocation processingof FIG. 5, similarly as in the processing of FIG. 4 described above, ifany of the distances between the base stations 11 ₁ to 11 _(n) and themobile terminal device 22 is less than or equal to a prescribed distance(d33min), the communication channel far from the frequency band that canbe used by the radio communication system 10 is allocated as thecommunication channel for the uplink from the mobile terminal device 22.

Consequently, in this communication channel allocation processing, thereduction of the interferences caused to the base stations 11 ₁ to 11_(n) of the radio communication system 10 can also be realized easily.

Also, in this communication channel allocation processing, theallocation of the communication channel for the uplink from the mobileterminal device 22 can be made according to the distance between eachone of the base stations 11 ₁ to 11 _(n) and the mobile terminal device22. Consequently, it is possible to realize the communication channelallocation that accounts for the actual utilization state of radiosignals, for example, so that it is possible to contribute to theimprovement of the spatial utilization efficiency of radio signals.

Note that the above description is directed to the case where thefrequency band to be used by the radio communication system 10 is lowerthan the frequency band to be used by the radio communication system 20,but this relationship can be reversed. In such a case, it suffices toswitch the communication channels to be allocated at the step S14 andthe step S16 in FIG. 4 described above, and switch the communicationchannels to be allocated at the step S32 and the step S34 in FIG. 5described above.

Note also that, in the above description, the present invention has beendescribed from a viewpoint of reducing interferences to be caused by theradio communication system 20 to the radio communication system 10, butthe present invention is also applicable to the case of reducinginterferences to be caused by the radio communication system 10 to theradio communication system 20.

For example, FIG. 1 described above is directed to the case where thetransmission output of the base stations 11 ₁ to 11 _(n) of the radiocommunication system 10 is extremely smaller than the transmissionoutput of the base stations 21 ₁ to 21 _(m) of the radio communicationsystem 20 and the cells 13 ₁ to 13 _(n) are contained within the cell 23₁, but there can be cases where the transmission output of the basestations 21 ₁ to 21 _(m) is smaller than the transmission output of thebase stations 11 ₁ to 11 _(n). In such cases, the cells 23 ₁ to 23 _(m)are going to be contained within the cell 13 ₁.

Note also that the spurious components such as higher harmoniccomponents, the intermodulation components, etc., that are caused by thenon-linearity of the amplifier, for example, are also generated from thebase stations 11 ₁ to 11 _(n) and the mobile terminal device 12 of theradio communication system 10 as well.

When these spurious components from the radio communication system 10are generated within the frequency band to be used as the uplinkfrequency by the radio communication system 20, the interferences willbe caused to the radio communication system 20.

For this reason, under such a condition, the radio communication controlunit 14 on the radio communication system 10 side is formed similarly asthe radio communication control unit 24 described above. Using thisradio communication control unit 14, the communication channelallocation processing similar to that of FIG. 4 or FIG. 5 describedabove is carried out when the base stations 21 ₁ to 21 _(m) exist withinthe cell 13 ₁, for example.

In this way, it is possible to detect a state that can potentially causeinterferences from the radio communication system 10 to the radiocommunication system 20 within the cell 13 ₁ easily, and realize thereduction of interferences to be caused from the radio communicationsystem 10 to the radio communication system 20 easily.

According to one communication channel allocation method of the firstembodiment, a distance between a first radio base station and a secondradio base station is detected, and when the detected distance is lessthan a first threshold, a distance between the second radio base stationand a mobile terminal device is detected, and when the detected distanceis less than a second threshold, a communication channel of a frequencyfar from a first frequency band is allocated to communications betweenthe second radio base station and the mobile terminal device.

In this way, the reduction of interferences to be caused to the firstradio base station by the communications between the second radio basestation and the mobile terminal device can be realized easily.

Also, by carrying out such a communication channel allocation, it ispossible to relax the characteristic required to the filter of themobile terminal device. Consequently, it is possible to contribute tothe reduction of the size and the power consumption of the mobileterminal device.

Also, according to another communication channel allocation method ofthe first embodiment, a distance between a first radio base station anda mobile terminal device is detected, and when the detected distance isless than a prescribed threshold, a communication channel of a frequencyfar from a first frequency band is allocated to communications betweenthe second radio base station and the mobile terminal device.

In this way, the reduction of interferences to be caused to the firstradio base station by the communications between the second radio basestation and the mobile terminal device can be realized easily.

Also, by carrying out such a communication channel allocation, it ispossible to relax the characteristic required to the filter of themobile terminal device. Consequently, it is possible to contribute tothe reduction of the size and the power consumption of the mobileterminal device.

In addition, it is possible to realize the communication channelallocation that accounts for the actual utilization state of radiosignals, according to the distance between the first radio base stationand the mobile terminal device, so that it is possible to contribute tothe improvement of the spatial utilization efficiency of radio signals.

Referring now to FIG. 6 to FIG. 14, the second embodiment of the radiocommunication system according to the present invention will bedescribed in detail.

In the first embodiment described above, when a covered area of a basestation of one radio communication system contains a plurality of basestations of another radio communication system under an environment inwhich two radio systems using close frequency bands exist, a frequencyof a channel to be used for communications between the base station ofthe one radio communication system and the mobile terminal device iscontrolled according to a distance between the base station of the oneradio communication system and the base station of the another radiocommunication system and a distance between the base station of the oneradio communication system and the mobile terminal device, so as toreduce interferences to be caused to the another radio communicationsystem while avoiding the complication of the device configuration.

However, in this communication channel allocation method, theappropriate channel allocation becomes difficult under an environment inwhich a cell of the base station of the one radio communication systemcontains many base stations of the another radio communication systembecause there are so many base stations of the another radiocommunication system whose distances should be accounted for the purposeof the channel control. As a result, there is some possibility forcausing interferences to the another radio communication system. Thesecond embodiment is directed to the communication channel allocationmethod for resolving this problem.

The present invention is applicable to the communication channelallocation, the communication control, etc., in a communication systemoperated under an environment in which another communication systemusing the close frequencies is existing, as shown in FIG. 6, forexample.

The other radio communication system 110 has a plurality of basestations 111 ₁ to 111 _(n) for providing the communication services, forexample, and a mobile terminal device 112 for utilizing thecommunication services provided by the base stations 111 ₁ to 111 _(n).

The radio communication system 110 is the PHS (Personal HandyphoneSystem), for example, which uses radio signals of 1.9 GHz band (1.89365to 1.91945 GHz), for example, for the communications between the basestations 111 ₁ to 111 _(n) and the mobile terminal device 112. Also,this radio communication system 110 uses the TDMA (Time DivisionMultiple Access) scheme in order to carry out communications between onebase station and a plurality of mobile terminal devices. For channelsfor carrying out communications by such a TDMA scheme, a frequency bandof 300 KHz is used per one channel, for example.

For the base stations 111 ₁ to 111 _(n), respective areas (cells) 113 ₁to 113 _(n) are allocated. Also, the base stations 111 ₁ to 111 _(n) areconnected through a communication line.

Each one of the base stations 111 ₁ to 111 _(n) provides a connectionservice for the Internet, a wired communication network, another radiocommunication network, etc. (which will be simply referred to as acommunication service hereafter), for example, with respect to themobile terminal device 112 within a corresponding one of the cells 113 ₁to 113 _(n).

Also, the radio communication system 120 has a plurality of basestations 121 ₁ to 121 _(m) for providing the communication services, amobile terminal device 122 for utilizing the communication servicesprovided by the base stations 121 ₁ to 121 _(m), and a radiocommunication control unit 124 for carrying out a control of radiocommunications between the base stations 121 ₁ to 121 _(m) and themobile terminal device 122.

In the radio communication system 120, a plurality of channels withdifferent frequencies are provided, for example, in order to carry outcommunications between one base station 121 and a plurality of mobileterminal devices 122, and radio signals of 2 GHz band immediately abovethe 1.9 GHz band is used, for example, for the communications betweenthe base stations 121 ₁ to 121 _(m) and the mobile terminal device 122.It is also possible to realize the TDD (Time Division Duplex) bydividing one frequency channel into uplink and downlink time-slots. Itis also possible to divide one frequency channel into more than twotime-slots to provide a plurality of logical channels that can be usedfor communications with a plurality of mobile terminal devices 122.

For the base stations 121 ₁ to 121 _(m), respective areas (cells) 123 ₁to 123 _(m) are allocated. Also, each one of the base stations 121 ₁ to121 _(m) is connected to the radio communication control unit 124through a wired or wireless communication line.

Each one of the base stations 121 ₁ to 121 _(m) provides a connectionservice for the Internet, a wired communication network, another radiocommunication network, etc. (which will be simply referred to as acommunication service hereafter), for example, with respect to themobile terminal device 122 within a corresponding one of the cells 123 ₁to 123 _(m), through the radio communication control unit 124.

Here, the radio communication system 110 uses radio signals with smallertransmission power compared with the radio communication system 120. Forexample, the transmission output of the mobile terminal device 112 ofthe radio communication system 110 is about 10 mW, for example, which isextremely small compared with the transmission output of the mobileterminal device 122 of the radio communication system 120. Also, thetransmission power of the base stations 111 ₁ to 111 _(n) is smallcompared with the transmission power of the base stations 121 ₁ to 121_(m).

For this reason, the cells 113 ₁ to 113 _(n) corresponding to the basestations 111 ₁ to 111 _(n) are smaller than the cells 123 ₁ to 123 _(m)corresponding to the base stations 121 ₁ to 121 _(m), and a plurality ofcells 113 ₁ to 113 _(n) are arranged within the cell 123 ₁.

Note that the number of the mobile terminal devices 112 and 122 are notlimited to those shown in FIG. 6 and it is possible to use arbitrarynumber of mobile terminal devices 112 and 122 within a range of thechannels allocated to each one of the base stations 111 ₁ to 111 _(n)and the base stations 121 ₁ to 121 _(m).

The radio communication control unit 124 has a configuration shown inFIG. 7, for example, which has a memory 125 for storing information suchas information (location information) indicating a location of themobile terminal device 122 of each user, a base station control unit 126for controlling operations of the base stations 121 ₁ to 121 _(m), anexchange unit 127 for controlling communications between the basestations 121 ₁ to 121 _(m) and a network 130 or the like, a radiochannel control unit 128 for controlling communication channels to beused for communications between the base stations 121 ₁ to 121 _(m) andthe mobile terminal device 122, and a call processing control unit 129for carrying out a control of a call termination for the mobile terminaldevice 122 or a call originating from the mobile terminal device 122.

The memory 125 stores a terminal location table 125 a for indicating thelocation information that indicate locations and the currently usedchannel group (chA, chB), etc., of the mobile terminal devices 122 thatare using the radio communication system 120, and a table (base stationinformation table) for indicating information such as locations andcommunication channels in use of the base stations 121 ₁ to 121 _(m).

The location information of each mobile terminal device 122 stored inthe terminal location table 125 a is expressed by a combination ofX-coordinate and Y-coordinate (Xi, Yi [i=1, 2, . . . , x]) within theservice providing region for providing the communication services withrespect to the mobile terminal device 122, for example. The mobileterminal device 122 obtains its own location according to strengths ofradio signals from a plurality of base stations 121 ₁ to 121 _(m) andlocations of these base stations 121 ₁ to 121 _(m), for example, andsupplies it to the radio communication control unit 124 along with anidentification information (user ID unique to the individual mobileterminal device 122, for example: Ui [i=1, 2, . . . , x]) assigned tothat mobile terminal device 122. The radio communication control unit124 stores the supplied identification information and locationinformation into the terminal location table 125 a through the basestation control unit 126 and the radio channel control unit 128.

Alternatively, it is also possible to provide a location detection unitsuch as the so called GPS (Global Positioning System) to the mobileterminal device 122 and supplies the location of the mobile terminaldevice 122 detected by this location detection unit to the radiocommunication control unit 124 similarly as described above. Theinformation indicating the location from the mobile terminal device 122is supplied to the radio communication control unit 124 at a prescribedtime interval, for example, such that the location of the mobileterminal device 122 stored in the terminal location table 125 a isregularly updated accordingly.

Now, in the radio communications, it is preferable to use thetransmission power that is minimum necessary in order to preventinterferences and mixing. On the other hand, in order to receive signalsfrom the mobile terminal device 122 surely at the base station 121,there is a need to make the signal to interference ratio (SIR) above aprescribed amount. For this reason, this radio communication system 120is made to control the transmission power of the mobile terminal device122 to an appropriate value.

The SIR is different depending on the power observed at the receivingside station and the power of noises. The noise power is obtained bymeasuring the power of components other than the channels actually usedfor communications, for example.

Also, the power observed at the receiving side station is differentdepending on the spatial propagation loss of radio signals, so that itis changed according to a distance between the transmitting station andthe receiving station. When the mobile terminal device 122 is thetransmitting station, the base station 121 ₁ is the receiving station,and the noise power is the same for both, in order to make the power tobe received by the base station 121 ₁ constant, there is a need to makethe transmission power of the mobile terminal device 122 large when thedistance between the mobile terminal device 122 and the base ^Ction 121₁ is large and to make the transmission power of the mobile terminaldevice 122 small when the distance between the mobile terminal device122 and the base station 121 ₁ is small.

For this reason, in this radio communication system 120, thetransmission power (Pi [i=1, 2, . . . , x]) of the mobile terminaldevice 122 is controlled to make a constant SIR for signals from themobile terminal device 122 that is carrying out communications with eachof the base stations 121 ₁ to 121 _(m). The transmission power controlof the mobile terminal device 122 is realized as the base station 121 ₁supplies a command for controlling the power to the mobile terminaldevice 122 and the mobile terminal device 122 controls the transmissionpower according to this command, for example. In this way, thetransmission power of the mobile terminal device 122 is controlled to belarge when the distance Di is large and the transmission power of themobile terminal device 122 is controlled to be small when the distanceDi is small.

As described above, the radio communication system 110 and the radiocommunication system 120 are using close frequencies. Also, as describedabove, the transmission output of the mobile terminal device 112 of theradio communication system is small compared with the transmissionoutput of the mobile terminal device 122 of the radio communicationsystem 120, so that there are cases where the large noises are caused tothe radio communication system 110 side even when the components outsidethe prescribed frequency band are attenuated to a sufficient level atthe radio communication system 120 side.

Also, because the transmission power of the mobile terminal device 122is larger than the transmission power of the mobile terminal device 112,there are cases where the so called receiver blocking occurs at thereceiver of the base stations 111 ₁ to 111 _(n) to lower the referencesensitivity, depending on the conditions.

More specifically, the interference power due to radio signals from themobile terminal device 122 that are received by the base stations 111 ₁to 111 _(n) in a vicinity of the base station 121 ₁ varies according tothe transmission power of the mobile terminal device 122 and thedistances between the mobile terminal device 122 and the base stations111 ₁ to 111 _(n). As described above, the transmission power of themobile terminal device 122 is controlled to be large when the distanceDi between the mobile terminal device 122 and the base station 121 ₁ islarge, and small when the distance Di is small. Consequently, theinterference power received from the mobile terminal device 122 by thebase stations 111 ₁ to 111 _(n) in a vicinity of the base station 121 ₁is large when the distance Di between the mobile terminal device 122 andthe base station 121 ₁ is large, and small when the distance Di issmall.

Also, the interference power due to radio signals from the mobileterminal device 122 that are received by the base stations 111 ₁ to 111_(n) in a vicinity of the base station 121 ₁ has correlation with afrequency of the channel used for communications between the mobileterminal device 122 and the base station 121 ₁ and frequencies of thechannels used by the base stations 111 ₁ to 111 _(n). The interferencepower is large when these frequencies are close, and the interferencepower is small when they are separated.

As shown in FIG. 8, for example, the radio communication system 120 usesthe frequency band of 2 GHz band immediately above the frequency band(1.9 GHz band, for example) used by the radio communication system 10.Also, as shown in FIG. 8, a frequency band (guard band) that is not tobe used by either radio communication system is provided between thefrequency band to be used by the radio communication system 110 and thefrequency band to be used by the radio communication system 120. Thisguard band has a bandwidth of 5 MHz, for example. Note that FIG. 8 showsthe spectrum intensities for the radio communication system 110 and theradio communication system 120 in different scales, for the sake ofmaking this relationship of frequencies easily comprehensible.

When the frequency close to the frequency band to be used by the radiocommunication system 110 is used between the mobile terminal device 122and the base station 121 ₁, the interference power to be received by thebase stations 111 ₁ to 111 _(n) in a vicinity of the base station 121 ₁becomes large. Conversely, when the frequency far from the frequencyband to be used by the radio communication system 110 is used betweenthe mobile terminal device 122 and the base station 121 ₁, theinterference power to be received by the base stations 111 ₁ to 111 _(n)in a vicinity of the base station 121 ₁ becomes small. This is due tothe fact that the strength of the spurious components such as higherharmonic components, the intermodulation components, etc., that arecaused by the non-linearity of the amplifier of the radio communicationdevice or the like becomes lower as they are more separated from thecarrier frequency.

As described above, the interference power received by the base stations111 ₁ to 111 _(n) also varies according to the distance between themobile terminal device 122 and the base station 121 ₁. For this reason,this radio communication system 120 is made such that a channel of afrequency far from the frequency band to be used by the radiocommunication system 110 is allocated to communications between themobile terminal device 122 and the base station 121 ₁ when the distancebetween the mobile terminal device 122 and the base station 121 ₁ islarge, and a channel of a frequency close to the frequency band to beused by the radio communication system 110 is allocated tocommunications between the mobile terminal device 122 and the basestation 121 ₁ when the distance between the mobile terminal device 122and the base station 121 ₁ is small.

Such a channel allocation can be realized by judging the frequency ofthe channel used by each mobile terminal device 122, and controlling thechannel allocation by the radio channel control unit 128, for example.

In order to reduce the control load, it is also possible to divide thechannels that can be used by the radio communication system 120 into agroup of those close to the frequency band to be used by the radiocommunication system 110 and a group of those far from the frequencyband to be used by the radio communication system 110, such thatavailable channel within each group (channel group) is allocated. Morespecifically, as shown in FIG. 8 described above, p channels that can beused in this radio communication system 120 are divided into a channelgroup close to the frequency band to be used by the radio communicationsystem 110 (ChA[chA1, chA2, . . . , chAk]) and a channel group far fromthe frequency band to be used by the radio communication system 110(ChB[chB1, chB2, . . . ]).

In order to reduce the control load further, it is also possible to amethod in which the number (N) of all the mobile terminal devices 122that are carrying out communications with the base station 121 ₁ whichis carrying out communications with the channel allocation controltarget mobile terminal device 122, the interference power (I_(all)) ofthe signals other than the transmission signals from the channelallocation control target mobile terminal device 122 is measured,channel allocation rates (α1, α2) for the channel group ChA and thechannel group ChB are obtained according to this interference powerI_(all) and the number N described above, a prescribed threshold(channel switching distance Dre) is set up in advance according to thesechannel allocation rates as shown in FIG. 9, for example, a channel inthe channel group ChA is allocated when the distance Di between themobile terminal device 122 and the base station 121 ₁ is less than thechannel switching distance Dre, or a channel in the channel group ChB isallocated when the distance Di is greater than or equal to the channelswitching distance Dre.

Such a communication channel allocation processing is carried outaccording to the procedure shown in FIG. 10, for example, such that whenthe mobile terminal device 122 generates a call with respect to the basestation 121 ₁, the processing is carried out from the step S101 of FIG.10. Note that, in the following, the case where a call is generated withrespect to one base station 121 ₁ will be described for the sake ofsimplicity, but there can be cases where one mobile terminal device 122generates calls with respect to a plurality of base stations 121 whenthe communication scheme capable of using a plurality of frequencies isused.

In the terminal location table 125 a in the memory 125 described above,the mobile terminal devices 122 within the cells 123 ₁ to 123 _(m) ofthe base stations 121 ₁ to 121 _(m) and the channel used by each mobileterminal device 122 are stored separately. The radio channel controlunit 128 obtains the number N of the mobile terminal devices 122 withinthe cell 123 ₁ of the base station 121 ₁ according to this table (stepS101).

When the number N of the mobile terminal devices 122 within the cell 123₁ is obtained, the radio channel control unit 128 measures the total(interference power) I_(all) of the received power of signals other thanthose of the mobile terminal device 122 that has generated the call(step S102). The interference power I_(all) so measured is supplied tothe radio channel control unit 128 through the base station control unit126, for example.

When the interference power I_(all) is supplied, the radio channelcontrol unit 128 obtains the allocation rates (α1, α2) of the channelgroups ChA and ChB at this base station 121 ₁ according to the number Nof the mobile terminal devices 122 and the interference power I_(all)obtained in the above (step S103). Here, α1 and α2 are positive valuesand have a relationship of α1+α2=1.

These rates α1 and α2 are determined according to a predeterminedfunction for optimizing the capacities of the radio communication system110 and the radio communication system 120 when N and I_(all) takeparticular values, for example.

This function is obtained by the computer simulation by setting thecommunication conditions appropriately, for example. Else, this functionmay be set up as a empirical formula obtained by the experiment usingthe actual communications, for example.

When the allocation rates α1 and α2 for the channel groups ChA and ChBare obtained, the radio channel control unit 128 determines the distance(channel switching distance) Dre for carrying out the switching of thecommunication channels (step S104).

This channel switching distance Dre is a value that satisfies arelationship of Dre²:R²−Dre²=α1:α2 where R is a radius of the cell 123,as shown in FIG. 9 described above.

In FIG. 9 described above, within the cell 123 ₁, the area of a regionfor which the distance from the base station 121 ₁ is less than or equalto the channel switching distance Dre and the area of a region for whichthe distance from the base station 121 ₁ is greater than the channelswitching distance Dre and less than or equal to the cell radius R canbe obtained as π·Dre² and π·R²−π·Dre². Consequently, the ratio of theareas of these regions is Dre²:R²−Dre². The channels in the channelgroups ChA and ChB are to be allocated respectively to communicationsbetween the mobile terminal devices 122 within these regions and thebase station 121 ₁, the channel allocation rates with respect to themobile terminal devices 122 within the cell 123 ₁ becomesα1:α2=Dre²:R²−Dre². Consequently, by setting the channel switchingdistance Dre that satisfies the above described relationship, thechannel allocation rates for the channel groups ChA and ChB within thecell 123 ₁ can be α1 and α2 respectively.

When such a channel switching distance Dre is obtained, the radiochannel control unit 128 checks whether the location information of themobile terminal device 122 that has generated the call is registered inthe terminal location table 125 a or not (step S105), and when it isregistered, the radio channel control unit 128 obtains the distance Dibetween the base station 121 ₁ and the mobile terminal device 122 andregisters it into the terminal location table 125 a (step S108).

When the location information of the mobile terminal device 122 is notregistered in the terminal location table 125 a, the radio channelcontrol unit 128 requests an acquisition of the location information ofthe mobile terminal device 122 (step S106), registers the acquiredlocation information of the mobile terminal device 122 into the terminallocation table 125 a, and executes the processing of the step S108.

When the distance Di between the base station 121 ₁ and the mobileterminal device 122 is obtained, the radio channel control unit 128compares it with the channel switching distance Dre obtained in theabove (step S109).

When the distance Di between the base station 121 ₁ and the mobileterminal device 122 is less than or equal to the channel switchingdistance Dre, the transmission power of the mobile terminal device 122is relatively low as a result of the transmission power control asdescribed above. For this reason, in such a case, even if the channel ofa frequency close to the frequency band to be used by the radiocommunication system 110, i.e., a channel in the channel group ChA, isallocated to communications between the mobile terminal device 122 andthe base station 121 ₁, the interference power observed at the basestations 111 ₁ to 111 _(n) in a vicinity of the base stations 121 ₁ willbe relatively small, and the influence of the interferences due to thetransmission signals of the mobile terminal device 122 will be small.

Consequently, when the distance Di between the base station 121 ₁ andthe mobile terminal device 122 is less than or equal to the channelswitching distance Dre, the radio channel control unit 128 allocatesavailable channel in the channel group ChA to communications between themobile terminal device 122 and the base station 121 ₁ (step S110).

On the other hand, when the distance Di between the base station 121 ₁and the mobile terminal device 122 is greater than the channel switchingdistance Dre, the transmission power of the mobile terminal device 122is relatively high. For this reason, in such a case, if the channel of afrequency close to the frequency band to be used by the radiocommunication system 110, i.e., a channel in the channel group ChA, isallocated to communications between the mobile terminal device 122 andthe base station 121 ₁, the interference power observed at the basestations 111 ₁ to 111 _(n) in a vicinity of the base stations 121 ₁ willbe relatively large, and the influence of the interferences due to thetransmission signals of the mobile terminal device 122 will be large.

Consequently, when the distance Di between the base station 121 ₁ andthe mobile terminal device 122 is greater than the channel switchingdistance Dre, the radio channel control unit 128 allocates availablechannel in the channel group ChB to communications between the mobileterminal device 122 and the base station 121 ₁ (step S111).

In this radio communication system 120, it is possible to reduce theinfluence of the interferences to be caused to the radio communicationsystem 110 by carrying out the channel allocation as described above.

Now, when the mobile terminal device 122 moves, the distance Di betweenthe base station 121 ₁ and the mobile terminal device 122 changes. Theradio channel control unit 128 regularly acquires the locationinformation of the mobile terminal device 122 and updates the locationinformation in the terminal location table 125 a, for example.Alternatively, the mobile terminal device 122 transmits a new locationinformation to the radio channel control unit 128 upon detecting themoving of the mobile terminal device 122 itself, and the radio channelcontrol unit 128 updates the location information in the terminallocation table 125 a upon receiving it.

There can be a case where the relationship of the distance Di betweenthe base station 121 ₁ and the mobile terminal device 122 and thechannel switching distance Dre changes as a result of the moving of themobile terminal device 122. In such a case, there is a need to switchthe channels to be used for communications between the mobile terminaldevice 122 and the base station 121 ₁.

FIG. 11 shows the procedure for such a channel switching processing,where the processing is started from the step S121 of FIG. 11 when thelocation information of the mobile terminal device 122 stored in theterminal location table 125 a is changed as the mobile terminal device122 has moved.

First, the radio channel control unit 128 obtains the current distanceDi′ between the mobile terminal device 122 and the base station 121 ₁from the updated new location information of the mobile terminal device122 (step S121).

Then, the radio channel control unit 128 obtains obtains the number N ofthe mobile terminal devices 122 within the cell 123 ₁ of the basestation 121 ₁ (step S122), obtains the interference power I_(all) at thebase station 121 ₁ (step S123), obtains the allocation rates α1 and α2of the channel groups ChA and ChB from N and I_(all) (step S124), andobtains the channel switching distance Dre (step S125), similarly as thesteps S101 to S104 of FIG. 10 described above.

When the channel switching distance Dre is obtained, the radio channelcontrol unit 128 compares the distance Di′ between the base station 121₁ and the mobile terminal device 122 obtained in the above with thechannel switching distance Dre (step S126).

When the distance Di′ between the base station 121 ₁ and the mobileterminal device 122 is less than or equal to the channel switchingdistance Dre, the radio channel control unit 128 checks whether thecurrently allocated channel is a channel in the channel group ChB or not(step S127), and if it is a channel in the channel group ChB, the radiochannel control unit 128 commands the switching to available channel inthe channel group ChA (step S128). If the currently allocated channel isa channel in the channel group ChA, it is left unchanged. In this way, astate in which a channel in the channel group ChA is allocated tocommunications between the mobile terminal device 122 and the basestation 121 ₁ is realized.

On the other hand, when the distance Di′ between the base station 121 ₁and the mobile terminal device 122 is greater than the channel switchingdistance Dre, the radio channel control unit 128 checks whether thecurrently allocated channel is a channel in the channel group ChA or not(step S129), and if it is a channel in the channel group ChA, the radiochannel control unit 128 commands the switching to available channel inthe channel group ChB (step S130). If the currently allocated channel isa channel in the channel group ChB, it is left unchanged. In this way, astate in which a channel in the channel group ChB is allocated tocommunications between the mobile terminal device 122 and the basestation 121 ₁ is realized.

When the above processing is finished, the radio channel control unit128 updates the distance Di in the terminal location table 125 a by thecurrent distance Di′ obtained in the above.

By this switching processing, even when the mobile terminal device 122has moved, the channel between the mobile terminal device 122 and thebase station 121 ₁ can be maintained appropriately.

In the first embodiment described above, as shown in FIG. 12, forexample, the channel to be used in one communication system 150 isselected according to the distance between the base station 141 ofanother communication system 140 and the mobile terminal device 152 ofthe one communication system.

This first embodiment is effective in reducing interferences to becaused to the base station 141 by the mobile terminal device 152 whenthe cell 143 of the base station 141 in the another communication system140 and the cell 153 of the base station 151 in the one communicationsystem 150 are nearly the same as shown in FIG. 13, for example, or whenthe sizes of the cell 143 and the cell 153 are not so different as shownin FIG. 14, for example.

However, under the circumstance as shown in FIG. 6 described above wherethe cell 113 in the another communication system is extremely smallcompared with the cell 123 in the one communication system and manycells 113 are contained within the cell 123, there can be cases wherethe another base station 111 exists near the mobile terminal device 122even if the base station 111 in a vicinity of the base station 121 andthe mobile terminal device 122 are separated. In such cases, there issome possibility for causing interferences to the base station 111 nearthe mobile terminal device 122 even if interferences are not caused tothe base station 111 in a vicinity of the base station 121.

For this reason, in this radio communication system 120, the channelallocation rates α1 and α2 for the close channel group ChA and the farchannel group ChB with respect to the frequency band to be used by theradio communication system 110 are determined according to the totalreceived power (interference power I_(all)) of signals other than thoseof the channel allocation target mobile terminal device 122 at the basestation 121 ₁ and the total number N of the mobile terminal devices 122within the cell 123 ₁ of the base station 121 ₁ with respect to whichthe transmission power of the mobile terminal device 122 is to becontrolled, and the channel switching distance Dre is obtained accordingto them. In addition, in this radio communication system 120, a channelin which one of the channel groups ChA and ChB is to be allocated isdetermined according to a result of the comparison of this channelswitching distance Dre with the distance Di between the base station 121₁ and the mobile terminal device 122.

By carrying out such a channel allocation, it is possible to reduceinterferences to be caused to the another radio communication system 110easily.

Note that the above description is directed to a configuration in whichthe radio communication system 120 simply multiplexes a plurality ofchannels by FDMA, but it is also possible to multiplex the channels byCDMA in which signals in each frequency channel is spread coded. In thiscase, the bandwidth of each channel to be used by the radiocommunication system 120 is about 5 MHz, for example, but the frequencychannel allocation processing can be carried out similarly as describedabove.

Note also that the above description is directed to the case where thefrequency band to be used by the radio communication system 120 is athigher frequencies than the frequency band to be used by the radiocommunication system 110, but the present invention is also applicableto the case where the frequency relationship among the frequency bandsis reversed, by appropriately changing the processing according to thefrequency relationship.

Thus, in this second embodiment, allocation rates for channels to beused for communications between a second radio base station and a mobileterminal device are obtained according to a number of mobile terminaldevices that are carrying out communications with the second radio basestation and a total received power (interference power) of signals otherthan those from the mobile terminal device, while controlling a receivedpower of signals from the mobile terminal device as received by thesecond radio base station to be constant, a channel switching distanceto be a criterion for switching communication channels is obtainedaccording to the obtained allocation rates, a distance between thesecond radio base station and the mobile terminal device is obtained,and a communication channel of a frequency far from a first frequencyband is allocated to communications between the second radio basestation and the mobile terminal device when the obtained distance isgreater than the channel switching distance.

As described above, the received power of the signals from the mobileterminal device as received by the second radio base station iscontrolled to be constant, so that when the distance between the secondradio base station and the mobile terminal device is greater than thechannel switching distance, the transmission power of the mobileterminal device is high and it can be expected that the interferencepower to be given to the first radio base station is large.

For this reason, by carrying out the channel allocation according to thedistance between the second radio base station and the mobile terminaldevice as described above, it is possible to reduce interferences to becaused to the first radio base station easily. In particular, when acovered area (cell) of the first radio base station is small comparedwith the cell of the second radio base station, and many first radiobase stations are contained in the cell of the second radio basestation, the interferences to be caused to the first radio base stationwill depend on the transmission power of the mobile terminal device, sothat by carrying out the channel allocation as described above, it ispossible to reduce interferences easily.

Also, by carrying out such a communication channel allocation, it ispossible to relax the characteristic required to the filter of themobile terminal device. Consequently, it is possible to contribute tothe reduction of the size and the power consumption of the mobileterminal device.

In addition, it is possible to realize the communication channelallocation that accounts for the actual utilization state of radiosignals, so that it is possible to contribute to the improvement of thespatial utilization efficiency of radio signals.

It is also to be noted that, besides those already mentioned above, manymodifications and variations of the above embodiments may be madewithout departing from the novel and advantageous features of thepresent invention. Accordingly, all such modifications and variationsare intended to be included within the scope of the appended claims.

1. A communication channel allocation method for allocating acommunication channel to communications between a mobile terminal deviceand a second radio base station using a second frequency band which isclose to a first frequency band used by a first radio base station, thecommunication channel allocation method comprising the steps of:detecting a first distance between the second radio base station and thefirst radio base station; detecting a second distance between the secondradio base station and the mobile terminal device when the firstdistance is less than a first threshold; and allocating a communicationchannel of a frequency far from the first frequency band in the secondfrequency band to the communications between the second radio basestation and the mobile terminal device when the second distance is lessthan a second threshold.
 2. The communication channel allocation methodof claim 1, further comprising: allocating a communication channel of afrequency close to the first frequency band in the second frequency bandto the communications between the second radio base station and themobile terminal device when the second distance is greater than or equalto the second threshold.
 3. The communication channel allocation methodof claim 1, further comprising: a step for allocating a communicationchannel of a frequency close to the first frequency band in the secondfrequency band to the communications between the second radio basestation and the mobile terminal device when the second distance isgreater than or equal to the second threshold.
 4. A communicationchannel allocation method for allocating a communication channel tocommunications between a mobile terminal device and a second radio basestation using a second frequency band which is close to a firstfrequency band used by a first radio base station, the communicationchannel allocation method comprising the steps of: detecting a distancebetween the first radio base station and the mobile terminal device; andallocating a communication channel of a frequency far from the firstfrequency band in the second frequency band to the communicationsbetween the second radio base station and the mobile terminal devicewhen the distance is less than a prescribed threshold.
 5. Thecommunication channel allocation method of claim 4, further comprising:allocating a communication channel of a frequency close to the firstfrequency band in the second frequency band to the communicationsbetween the second radio base station and the mobile terminal devicewhen the distance is greater than or equal to the prescribed threshold.6. The communication channel allocation method of claim 4, furthercomprising: a step for allocating a communication channel of a frequencyclose to the first frequency band in the second frequency band to thecommunications between the second radio base station and the mobileterminal device when the distance is greater than or equal to theprescribed threshold.
 7. A communication control device for controllinga communication channel to be used for communications between a mobileterminal device and a second radio base station using a second frequencyband which is close to a first frequency band used by a first radio basestation, the communication control device comprising: a first distancedetection unit configured to detect a first distance between the secondradio base station and the first radio base station; a second distancedetection unit configured to detect a second distance between the secondradio base station and the mobile terminal device when the firstdistance is less than a first threshold; and a channel allocation unitconfigured to allocate a communication channel of a frequency far fromthe first frequency band in the second frequency band to thecommunications between the second radio base station and the mobileterminal device when the second distance is less than a secondthreshold.
 8. The communication control device of claim 7, wherein thechannel allocation unit is also configured to allocate a communicationchannel of a frequency close to the first frequency band in the secondfrequency band to the communications between the second radio basestation and the mobile terminal device when the second distance isgreater than or equal to the second threshold.
 9. A communicationcontrol device for controlling a communication channel to be used forcommunications between a mobile terminal device and a second radio basestation using a second frequency band which is close to a firstfrequency band used by a first radio base station, the communicationcontrol device comprising: a distance detection unit configured todetect a distance between the first radio base station and the mobileterminal device; and a channel allocation unit configured to allocate acommunication channel of a frequency far from the first frequency bandin the second frequency band to the communications between the secondradio base station and the mobile terminal device when the distance isless than a prescribed threshold.
 10. The communication control deviceof claim 9, wherein the channel allocation unit is also configured toallocate a communication channel of a frequency close to the firstfrequency band in the second frequency band to the communicationsbetween the second radio base station and the mobile terminal devicewhen the distance is greater than or equal to the prescribed threshold.11. A communication channel allocation method for allocating acommunication channel to communications between a mobile terminal deviceand a second radio base station using a second frequency band which isclose to a first frequency band used by a first radio base station, thecommunication channel allocation method comprising: a step for detectinga first distance between the second radio base station and the firstradio base station; a step for detecting a second distance between thesecond radio base station and the mobile terminal device when the firstdistance is less than a first threshold; and a step for allocating acommunication channel of a frequency far from the first frequency bandin the second frequency band to the communications between the secondradio base station and the mobile terminal device when the seconddistance is less than a second threshold.
 12. A communication channelallocation method for allocating a communication channel tocommunications between a mobile terminal device and a second radio basestation using a second frequency band which is close to a firstfrequency band used by a first radio base station, the communicationchannel allocation method comprising: a step for detecting a distancebetween the first radio base station and the mobile terminal device; anda step for allocating a communication channel of a frequency far fromthe first frequency band in the second frequency band to thecommunications between the second radio base station and the mobileterminal device when the distance is less than a prescribed threshold.